The use of rubber articles has always been subject to the problem of oxygen and ozone degradation. Over the years numerous solutions have been proposed to lessen or eliminate the destruction of the rubbery polymers by attack from ozone and/or oxygen. Typically antidegradants have been incorporated into the rubber compound to lessen or eliminate the degradation process.
More recently, several new approaches have been submitted to further enhance a rubber's ability to resist ozone and oxygen degradation. These new approaches include higher molecular weight materials, polymer bound materials and specific blends of antioxidants and/or antiozonants with waxes, rosins and the like. One problem that the prior art approaches have failed to solve is that the antiozonant at the rubber article surface is depleted after several years of service. This is especially true of articles such as truck tires that may be recapped or retreaded three or more times. In such use the rubber, especially the tire sidewall, must exhibit extended antiozonant protection so that the tire carcass may be used to its fullest potential.
In general, the approach of this invention is to place the antidegradant in a matrix of vulcanized rubber and grind the rubber into a microparticle with a diameter of 100 microns or less. The microparticles are compounded into rubber formulations that are used for tires and other rubber articles. The matrix material is selected based upon its higher solubility for the antidegradant than the rubber compound in which the microparticles are incorporated and thus the microparticles act as antidegradant reservoirs. A slow release of the antidegradant is achieved as the antidegradant diffuses from the microparticle reservoir into the rubber compound. This migration or diffusion provides a surface concentration of the antidegradant at an appropriate level. The critical or minimum concentration of the antidegradant needed to protect the rubber article from oxygen or ozone attack is thus maintained for longer periods of time. Through the use of the matrix-antidegradant of this invention, the effective service life of a rubber article is greatly enhanced.
In general, the present invention utilizes a controlled release system for the delivery of a rubber antidegradant as it is needed. The controlled release system or matrix system uses a polymer matrix that contains high levels of the antidegradant. The matrix polymer does not chemically disintegrate or erode, but rather releases the antidegradant to the bulk of the rubber article for migration to the surface through diffusion. One critical aspect of this invention resides in the discovery that a partitioning coefficient of at least 2 is necessary. The partitioning coefficient is a measure of the relative solubility of the antidegradant in the matrix polymer versus the stock or host polymer. One benefit of the matrix system is that it can facilitate handling. For example, a matrixed liquid may be handled much like a free flowing powder, while controlled release of the desired material may actually reduce the amount of active ingredient needed to do the job. In the pharmaceutical industry, controlled release systems have been used to sustain a minimum effective dose level for extended periods of time.
Through extensive research and effort, the inventors herein have developed a matrix system using specific components and specific processing parameters to provide enhanced protection to a rubber article from oxygen and ozone attack.
The use of granulated vulcanized rubber in rubber compounds has been known for some time as a method for recycling waste rubber. The use of recycled rubber has found limited acceptance since the physical properties of final product are usually less than desired. Further, some granulation or grinding methods use chemicals and energy intensive mechanical devices that substantially increases the cost.
U.S. Pat. Nos. 4,046,834 and 4,098,737 disclose a method for converting vulcanized rubber into finely divided vulcanized rubber which comprises 1) contacting the vulcanized rubber with a fatty acid; 2) adding solid alkali; 3) forming a dispersion with water using a disc mill; and 4) removing the finely divided rubber from the dispersion. U.S. Pat. Nos. 4,449,670; 4,449,674; 4,597,535; 4,469,284; 4,614,310: and 4,625,922, which are incorporated herein by reference, describe a comminuting apparatus for grinding vulcanized rubber and methods of obtaining microparticles of vulcanized rubber without the use of a softening agent, alkali and the like. The present invention preferably uses the microparticles produced through the apparatus and methods disclosed in these six U.S. Pat. Nos..
U.S. Pat. No. 4,092,285 discloses a composition of matter useful for rapidly and homogeneously dispersing a chemical in a rubber-or plastic compound, said composition comprising from about 60 to about 95 percent by weight of the chemical and from about 5 to about 40 percent by weight of a binder, said binder comprising at least one component selected from 1) liquids compatible with said rubber or plastic compound and with the other binder components; and 2) waxes having a sharp melting point in the range of about 55 to 80.degree. C.; and 3) a polymer having a molecular weight of at least about 50,000 which when combined with the other binder components produces a binder which is a non-tacky gel below about 53.degree. C. and returns to a liquid above about 53.degree. C..
The prior art does not suggest nor does it disclose a microparticle reservoir concept or matrix-antidegradant for stockpiling additional antidegradant necessary to extend the useful lifetime of rubber products. It is not possible to simply incorporate higher levels of antidegradants into the rubber compound without detrimentally impacting its physical properties. Further, high levels of free antiozonant only increases the initial surface concentration above the level required for effective protection without significantly extending the time prior to failure. A matrix-antidegradant or microparticle reservoir approach stockpiles needed antidegradant which is slowly released to the rubber compound when the compound's level of antiozonant is depleted below a critical concentration. This release of the antidegradant from the matrix polymer to the stock to be protected is accomplished through the proper selection of the matrix polymer with a partitioning coefficient of at least 2 with respect to the rubber stock to be protected.
The Applicants have discovered that the preparation of a matrix-antidegradant as described and claimed herein will survive Banbury mixing at temperatures in excess of 110.degree. C. and survive the curing process which achieves temperatures in excess of 150.degree. C.. Further, it was unexpectedly observed that the products prepared according to this invention were compatible with rubber formulations and provided protection for the useful lifetime of the tire or the rubber object. In addition, it was discovered that the matrix-antiozonant controls the rate at which the antiozonant becomes available for consumption at the surface of the rubber article and therefore, greatly extends the effective lifetime of the rubber article.